The present invention relates to a sheathed-element glow plug, for example, for starting a self-igniting combustion engine.
It is believed that sheathed-element glow plugs are known. To start a self-igniting combustion engine, an initial ignition of a fuel-air mixture may be required, which may be supplied by sheathed-element glow plugs positioned in a wall of a combustion chamber. The sheathed-element glow plugs include a heating pin, which may contact the fuel-air mixture to be ignited.
The heating pin may be produced from electrically conductive ceramic. In this context, the heating pin may have a defined electrical resistance, so that a heating current will flow when the heating pin is connected to a voltage source, which may produce a specific temperature in heating the heating pin, and which may be sufficient to ignite the fuel-air mixture.
For monitoring and controlling the operation of the self-igniting combustion engine, it may be advantageous to determine the heating pin temperature. For this purpose, the temperature of the heating pin may be derived from a measurement of the heating current flowing through the heating pin. The electrically conductive ceramics, of which the heating pins may be made, may have a positive temperature coefficient. Thus, since increasing temperature causes the resistance to increase, the heating current decreases, given a constant supply voltage. From this, the instantaneous temperature of the heating pin may be determined from the time characteristic of the heating current. However, it is believed to be disadvantageous that the temperature distribution over the length of the heating pin may vary considerably at equal heating current. For example, the temperature distribution may be a function of a rotatory speed, a load condition and/or cooling of the combustion engine. Temperature differences of up to, for example, 200xc2x0 C., may occur.
An exemplary sheathed-element glow plug according to the present invention permits a direct temperature measurement at the tip of a heating pin, without impairing the actual glowing function of the sheathed-element glow plug. Since the sheathed-element glow plug includes an integrated temperature sensor, the temperature of the heating pin may be determined both during active operation of the sheathed-element glow plug and during the passive set-up of the sheathed-element glow plug. This may permit an accurate measurement of the temperature, independently of the operating state of the self-igniting combustion engine.
In another exemplary embodiment according to the present invention, the temperature sensor is integrated directly into the heating pin. The heating pin may include, for example, a bore hole extending essentially axially, for accommodating the temperature sensor. The integration of the temperature sensor into the sheathed-element glow plug may be simple, and no additional construction space for the temperature sensor may be required, since the sensor is integrated inside the heating pin.
In yet another exemplary embodiment according to the present invention, the bore hole, which accommodates the temperature sensor, is positioned inside an insulating core of the heating pin, thereby permitting the temperature sensor to be positioned, without impairment of the actual glowing function of the heating pin.
In still another exemplary embodiment according to the present invention, the bore hole of the heating pin, which accommodates the temperature sensor, includes a groove with an open edge. This may permit the temperature sensor to be positioned directly adjacent to an outer circumferential wall of the heating pin, so that the temperature may be exactly measured, since the arrangement in the open edged recess obviates the need to consider the thermal transition resistance of the ceramic material of the heating pin.